Stars as Laboratories for Fundamental Physics - MPP Theory Group

Anomalous Stellar Energy Losses Bounded by Observations 61
Fig. 2.16. Observables in the color-magnitude diagram of a typical globular
cluster (here M15 after Sandage 1986). Depending on the metallicity, the
red-giant “bump” can also appear below the HB brightness.
In the color-magnitude diagram of a globular cluster (Fig. 2.3) the
stars arrange themselves in a characteristic pattern which is schematically
shown in Fig. 2.16. Each branch corresponds to a certain evolutionary
phase as discussed in Sect. 2.1. Typically, all stars in a
given cluster are coeval and have a fixed chemical composition; they
differ only in their mass. After formation, they all began their evolution
on the zero-age main-sequence as in Fig. 2.2. The more massive
stars evolve faster, become red giants and explode as supernovae
(M > ∼ 8 M ⊙ ), or become white dwarfs (M < ∼ 8 M ⊙ ). Recently, many
pulsars, the remnants of type II supernovae and thus of massive stars,
have indeed been found in globular clusters. The upper main sequence
(MS) is depleted of stars down to a limiting value below which they
have not had time to complete hydrogen burning. Therefore, the stellar
mass corresponding to the MS turnoff (TO) is a precise measure
of the cluster age. A typical value is 0.7−0.9 M ⊙ , depending on the
precise age and metallicity.
The color-magnitude diagram of a globular cluster represents an
“isochrone” of a stellar population. It shows the locus of coeval stars
with different initial masses. It is to be distinguished from the evolutionary
track of a single star (Fig. 2.6) which shows a star of a fixed
mass at different ages. However, the evolution beyond the MS is very